A representative material gas concentration control system for this kind of material evaporation system is a material gas concentration control system comprising a mass flow controller arranged in an inlet line that introduces a carrier gas, a constant temperature bath to keep a tank where a material liquid is stored at a constant temperature, and a pressure sensor that is arranged in an outlet line to output a mixed gas comprising a material gas and the carrier gas and that measures a pressure, namely a total pressure, of the mixed gas.
This material gas concentration control system evaporates the material liquid constantly at a saturated vapor pressure with a temperature of the material liquid kept constant so as to make a partial pressure of the material gas at a constant temperature and to control the flow of the carrier gas so as to make the total pressure measured by the pressure sensor at a constant temperature by the use of the mass flow controller. With this arrangement, since the concentration of a vapor component is expressed by the partial pressure of the vapor component divided by the total pressure of the system, the concentration of the vapor component, namely, the concentration of the material gas, is also considered to be constant because the partial pressure and the total pressure are constant.
However, even though the tank is kept at a constant temperature by the use of a constant temperature bath, since the saturated vapor pressure changes due to the drop of the temperature because of the heat of vaporization at a time of vaporizing the material liquid, and the partial pressure of the material gas also changes, the concentration of the material gas deviates from the desired concentration. In addition, when the amount of the material liquid changes, a contact period during which the carrier gas contacts the material liquid or a state of the carrier gas contacting the material liquid by means of bubbling changes, so that the material gas fails to achieve vapor-liquid equilibrium and saturate the vapor gas. As a result, the partial pressure of the material gas changes so that the concentration of the material gas deviates from the desired concentration.
Furthermore, even though the material liquid is vaporized at the saturated vapor pressure on a constant basis, in a case of trying to change the concentration of the material gas, it is necessary to change the saturated vapor pressure by changing the temperature in the tank. Since it usually takes a long time to change the temperature in the tank, the responsiveness of controlling the concentration of the material gas becomes poor.
As mentioned, in order to control the concentration of the material gas, since it is necessary to arrange a component in both the inlet line and the outlet line, a component mounting man-hour increases or component mounting becomes troublesome.
Patent document 1: U.S. Published Patent Application No. 2007/0254093
Patent document 2: Japanese Laid-Open Patent Publication No. 2003-257871